Respect the traditions and imagine that Alice wants to send 5 BTC to Bob
A small file, called transaction, is then created by Alice’s wallet application and is published on the network. This file, signed by Alice, contains: (1) the public keys that identify Alice and Bob, (2) the amount of 5 BTC that Alice wishes to transfer and (3) the references to the previous transactions that gave Alice the possession of these 5 BTC (which we will understand the reason for being in a moment). Thanks to the PKI, no one can pretend to be Alice, and any node (= machine) in the bitcoin price today network will be able to verify the authenticity of this transaction. As each transaction refers to the previous ones (see (3)), it is, in fact, the entire chain of possession of bitcoins, which is verifiable.
In reality, Bitcoin does not have an account book
Alice’s balance, for example, is not written anywhere but can be calculated from the chain of all transactions since the system was created, of which each node has a regularly updated copy. There remains the thorniest point: how to prevent Alice from being able to fraudulently send these same 5 BTCs to someone else after having already sent them to Bob? In a traditional banking system, such verification is easy because the central system defines a chronology, which makes it possible to establish the precedence of one transaction over another. In a P2P network like bitcoin price today on the other hand, the latency of the network prevents such verification.